Microwave window



June 4, 1968 E. J, COOK 3,387,237

MICROWAVE WINDOW Filed Dec. 27, 1965 FIG.I E v INVENTOR.

# EDWARD J. COOK ATTORNEY United States Patent 3,387,237 MICROWAVE WINDOW Edward J. Cook, South Hamilton, Mass., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Dec. 27, 1965, Ser. No. 516,499 9 Claims. (Cl. 333-98) The present invention relates in general to microwave windows and, more particularly, to an improved resonant window capable of passing high RF. power and which permits a smaller sized iris on one side thereof for RF. field concentration and which may be readily tuned. Such windows are especially useful in gas-filled discharged devices such as TR tubes, ATR tubes, pre TR tubes, duplexers, and the like as well as in microwave electron tubes such as klystrons and magnetrons.

Heretofore, R.F. windows have been made which included a dielectric slab forming the gas-tight wave permeable member sandwiched between a pair of centrally apertured metallic frame members defining an iris opening. It was also included in this prior design that the metal frame member on one side had a smaller aperture for passage of the wave energy, whereby the R.F. fields of the wave energy were concentrated (made higher in intensity) on the side of the window defined by the frame having the smaller opening. In this manner the strong fields associated with passage of high power through the window could be located as desired by facing the small opening of the window to one side or the other of the gas-tight wave permeable partition.

One of the problems with this prior design was that in order to provide a strong frame member to hold a pressure differential across the window during baking and brazing and to prevent fracture of the window due to the different coefiicients of thermal expansion between the frame members and the Window member, usually alumina ceramic, the frame member material was selected of a strong material and to have the same, or nearly the same, coefficient of thermal expansion as the window member. This meant that the frames were usually made of Kovar which is a relatively high strength material with a thermal coefficient of expansion near but not exactly that of glass and ceramic. However, at high temperatures obtained during tube processing and brazing and at high microwave power levels the frame member having the smaller opening is stronger than the other frame because of the larger dimensions of its metal annulus. This uneven frame strength on opposite sides of the thin ceramic partition which has a differential expansion than that of the frames produces a very strong bending moment because of the higher ceramic restraining force exerted by the stronger frame, causing the ceramic to fracture, thus producing failure of the window.

In the present invention, both apertured window frame members are sealed to the window member over substan' tially coextensive marginal areas of the window member. In addition, a thin yieldable ductile metallic lip portion is secured around the central aperture in one of the frame members to define the margins of the smaller iris opening for concentrating the RF. fields on one side of the window. In this manner both frame members have equal "ice The principal object of the present invention is the provision of an improved resonant window.

One feature of the present invention is the provision of a window having a pair of metallic frame members with different sized aligned apertures sealed to the window on opposite sides over substantially coextensive annular areas of the window, whereby thermal stress in the window is reduced.

Another feature of the present invention is the same as the preceding wherein the smaller aperture in the one frame member is defined by a yieldable ductile metallic lip portion.

Another feature of the present invention is the same as any one or more of the preceding features wherein the frame members are of non-ductile material such as Kovar, molybdenum, columbium, or tantalum and the yieldable lip portion is of a ductile metal, such as copper, silver or gold.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a longitudinal cross sectional view of a window assembly incorporating features of the present invention,

FIG. 2 is a transverse view of the structure of FIG. 1 taken along line 22 in the direction of the arrows;

FIG. 3 is a schematic equivalent circuit of the window of FIGS. 1 and 2,

FIG. 4 is an alternative view to that of FIG. 2 showing an alternative embodiment of the present invention, and

FIG. 5 is an enlarged detail view of a portion of FIG.

1 delineated by line 5-5.

Referring to FIGS. 1-3, there is shown a wave-propagating structure such as a rectangular waveguide 1. The waveguide 1 is shown with flanges 2 for connecting to a suitable device such as a source of microwave power or a gas-filled device such as a TR, pre TR, ATR, or duplexer, not shown. A window assembly 3 is disposed transversely across the waveguide 1 serving as a wave permeable gas-tight partition therein such that a pressure differential may be maintained across the window assembly 3. More specifically, a vacuum may be maintained on one side of the window while atmospheric pressure is maintained on the other, or some other gaseous fill material is maintained at a pressure other than vacuum.

The window assembly 3 comprises a pair of identical 7 relatively non-ductile metallic frame members 4 and 5 strengths, because the ductile lip contributes nothing to as of Kovar, molybdenum, columbium or tantalum transversely sealed across the waveguide 1 as by being brazed at their outer peripheries to the inside wall of the waveguide 1. The frames 4 and 5 are centrally recessed at 6 to receive an oval-shaped wave permeable window member 7 as of alumina ceramic. The ceramic is provided with a pair of ring-shaped metallized regions 8 of equal coextensive area on opposite sides thereof near the outer periphery or margin of the window member 7. The window member 7 is sandwiched as by being brazed between the two frame members 4 and 5 and held within the recessed portion 6. The radial extent of the metallized ring 8 is substantially coextensive with the identical recessed portions 6 of the frames 4 and 5. The frames 4 and 5 are brazed to the metallized region 8 of the window member 7 by conventional brazing techniques to form a gastight seal therebetween. The frame members 4 and 5 are provided with a pair of axially aligned oval-shaped centrally placed apertures 9 to define a resonant iris opening for passage of electromagnetic wave energy therethrough in the direction of the longitudinal axis of the guide 1.

An oval-shaped thin ring member 11 is sealed as by brazing to the inner lip portion of the aperture 9 in frame member 5. Ring 11 is preferably made of a thin yieldable ductible material as of copper, silver, or gold projecting inwardly from opening 9 to define the iris opening in frame 5, which iris opening is smaller than the iris opening formed by aperture 9 in frame 4. In this manner the effective iris opening for the window assembly 3 is defined by the smaller iris formed by the metallic ring 11.

The window assembly 3 forms a shunt resonant circuit element across the guide having a pass band characteristic corresponding to the center frequency of the iris determined by the dimensions of the opening formed by the center of the ring 11, see FIG. 3.

Window assemblies 3, to cover a range of different pass bands, may be readily constructed by using the same sized frame members 4 and 5 and merely substituting different sized rings 11, i.e., different radial extent, to define a series of windows operable over different pass bands within the pass band of the waveguide 1. Fine adjustments in the resonant frequency and thus pass band of the window assembly 3 may be made by inserting a tool, not shown, between the dialectric window member 7 and the deformable ring 11 and distorting ring 11 by pulling it away from the window or by pushing it closer to the window member 7 (see FIG. 5). This changes the capacitance of the Window as indicated in FIG. 3.

In a typical window design operable at C band, the window frame members 4 and 5 have a height of 0.980 and a width of 1.980" conforming to the inside dimensions of the C band rectangular waveguide 1. The oval apertures 9 are 0.563" high and 1.062" in width. The ceramic window member 7 is 1.250" wide and 0.750" high with the annular metallized rings having a width of about 0.080" metallized to a depth of aproximately 0.002". The thickness of the window 7 is approximately 0.030. The yieldable ductile ring member 11 is approximately 1.240 in width and 0.740" in outside height with an inside height of aproximately 0.563 and an inside width of approximately 1.062". The oval ring is preferably made of 0.005" thick half hard oxygen-free copper sheet. A window having the aforementioned typical dimensions had a 4% bandwidth at C band and readily passed 100 kw. peak and 500 watts average power without failure and was trimmable in frequency 80 megacycle's.

Although a preferred embodiment of the present invention has the lip portion 11 formed of a ring shape, the lip need not extend entirely around the aperture 9 in the frame 5 to define the smaller iris. More particularly, the ductile lip may comprise a tab 21 as shown in FIG. 4 which is brazed to the margin of aperture 9 and which tab 21 overlies the window 7, preferably extending across the window in the direction of the narrow dimension of the waveguide 1.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A microwave window assembly including, means forming a wave permeable window member for gas-tight partitioning a wave-supporting structure, means forming a pair of apertured metallic frame members sealed in a gas-tight manner to the margin of said window member and sandwiching said wave permeable member therebetween, said apertures in said frame members defining a pair of iris openings in said frame members, one of said iris openings being smaller than the other for concentrating the electric fields of the wave energy on that side of said window member, and a lip portion of said frame member which defines said smaller iris being yieldable and made of a more ductile material than its adjoining frame portion, whereby thermally produced stresses in said window member are reduced in use.

2. The apparatus according to claim 1 wherein both of said frame members are sealed to said window member over substantially equal coextensive marginal areas of said window member.

3. The apparatus according to claim 2 wherein said frame members are selected from the class consisting of Kovar, molybdenum, columbium, and tantalum and said lip portion is made of a material selected from the class consisting of copper, silver, and gold.

4. The apparatus according to claim 3 wherein said lip portion is made of copper and said frames are made of Kovar.

5. The apparatus according to claim 1 wherein said ductile lip portion is made of an annular shape.

6. The apparatus according to claim 5 wherein said frame members are made of a material having a thermal coefiicient of expansion aproximately equal to the thermal coefficient of expansion of said window members.

7. The apparatus according to claim 5 wherein said lip portion is spaced from said window member in overlying relation to permit insertion of a tool for trimming the resonant frequency of said window assembly.

8. The apparatus according to claim 1 including, a wave-supporting structure having said window member and frame members sealed transversely thereacross partitioning said wave-supporting structure into two regions on opposite sides of the window assembly capable of supporting different gas pressures therein.

9. The apparatus according to claim 1 wherein said yieldable ductile lip portion comprises a tab overlying said window member.

References Cited UNITED STATES PATENTS 2,407,069 9/1946 Fiske 33398 2,930,008 3/1960 Walsh 333-98 2,971,172 2/1961 Hamilton et a1. 333-98 ELI LIEBERMAN, Primary Examiner.

HERMAN KARL SAALBACH, Examiner.

L. ALLAHUT, Assistant Examiner. 

1. A MICROWAVE WINDOW ASSEMBLY INCLUDING, MEANS FORMING A WAVE PERMEABLE WINDOW MEMBER FOR GAS-TIGHT PARTITIONING A WAVE-SUPPORTING STRUCTURE, MEANS FORMING A PAIR OF APERTURED METALLIC FRAME MEMBERS SEALED IN A GAS-TIGHT MANNER TO THE MARGIN OF SAID WINDOW MEMBER AND SANDWICHING SAID WAVE PERMEABLE MEMBER THEREBETWEEN, SAID APERTURES IN SAID FRAME MEMBERS DEFINING A PAIR OF IRIS OPENINGS IN SAID FRAME MEMBERS, ONE OF SAID IRIS OPENINGS BEING SMALLER THAN THE OTHER FOR CONCENTRATING THE ELECTRIC FIELDS OF THE WAVE ENERGY ON THAT SIDE OF SAID WINDOW MEMBER, AND A LIP PORTION OF SAID FRAME MEMBER WHICH DEFINES SAID SMALLER IRIS BEING YIELDABLE AND MADE OF A MORE DUCTILE MATERIAL THAN ITS ADJOINING FRAME PORTION, WHEREBY THERMALLY PRODUCED STRESSES IN SAID WINDOW MEMBER ARE REDUCED IN USE. 